Inheritance: Gametes & Fertilisation
- Gametes are the sex cells of an organism
- For example, the sperm and egg (ovum) cells in humans
- The egg is larger than the sperm as most of its space contains food to nourish a growing embryo
- The sperm cell contains many mitochondria to release energy for its motion
- Gametes fuse during fertilization to form a zygote (fertilised egg cell)
- These sex cells are formed during meiosis and only have one copy of each chromosome and so are haploid cells
- For humans, that means the sperm and egg cells contain 23 single chromosomes in their nucleus (as opposed to diploid cells which contain 46 chromosomes, or 23 pairs)
- As there is only one chromosome from each homologous pair there is only one allele of each gene present
- This allele may be dominant, recessive or co-dominant
Sperm cell diagram
Egg cell diagram
The structure of human gametes - the sperm and egg
- Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles
- Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
- Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
- When a male and female gamete fuse their chromosomes are combined
- This means the resulting zygote is diploid
- The zygote contains two chromosomes of each type
- It will therefore have two alleles of each gene
- If the two alleles for a particular gene are the same then the genotype is described as homozygous
- If the two alleles for a particular gene are different then the genotype is described as heterozygous